How to calculate maximum available volume capacity with X-RAID and various HDDs?

[AGSowjet]

We bought our first RN104 a couple of years ago with two 3 (2,7) TB disks in RAID 1 mode. After a while we added two 4 (3,6) TB disks and continued using RAID 1, giving us a total capacity (data + snapshots + free space) as expected of 6,3 TB.

Last year we bought a second RN104 with two 4 (3,6) TB disks, using X-RAID in RAID5 mode by default, and got a total capacity of 3,6 TB. Now after adding two 8 (7,3) TB disks, we have 14,95 TB total capacity, smothelessly configured by automatic. This is perfect, isn't it? As we need storage more than speed, a greater capacity could not be achieved with security for a single disk failure, I guess.

The reason why I am writing here is because the FAQ und online docs are in many parts confusing, as options have changed with firmware upgrades and some docs refer to old versions… so there were Flex-RAID, X-RAID, X-RAID II, but RN104 with latest firmware 6.6.0 only has one button: X-RAID on or off. X-RAID seems to be the perfect choice for most scenarios though.

But I'm not sure whether it will make sense to reconfigure the first RN104 to X-RAID. I guess we could gain only 1 TB in capacity, because it will use the 4 TB disks for data (3,6*2=7,2) and both 3 TB disks for parity, actually wasting 2 TB, correct?

Any ideas how to get the maximum of our HDDs in two ReadyNAS 104?

[StephenB · Accepted solution]

The capacity rule for single redundancy is sum the disks and subract the largest.

2x4TB + 2x8TB gives you 16 TB capacity (14.55 TiB).  This is the maximum capacity possible if you want to protect against single disk failure.

If you switched your other NAS to use XRAID, 2x3TB+2x4TB would give you 10 TB (9.09 TiB). That's  3 TB more capacity then dual-RAID 1.

[StephenB · Accepted solution]

The capacity rule for single redundancy is sum the disks and subract the largest.

2x4TB + 2x8TB gives you 16 TB capacity (14.55 TiB).  This is the maximum capacity possible if you want to protect against single disk failure.

If you switched your other NAS to use XRAID, 2x3TB+2x4TB would give you 10 TB (9.09 TiB). That's  3 TB more capacity then dual-RAID 1.

[AGSowjet]

Awesome. I'll do that – will start backing up all contents of the first RN to the second RN immediately, then reconfigure the first. (Afaik, switching to X-RAID will destroy the volume.)